virtio-devices: vhost_user: blk: Cleanup device creation

Prepare the device creation so that it can be factorized in a follow up
commit.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

virtio-devices/src/vhost_user/mod.rs

@@ -84,6 +84,10 @@ pub enum Error {
     VhostUserSetSlaveRequestFd(vhost::Error),
     /// Add memory region failed.
     VhostUserAddMemReg(VhostError),
+    /// Failed getting the configuration.
+    VhostUserGetConfig(VhostError),
+    /// Failed setting the configuration.
+    VhostUserSetConfig(VhostError),
     /// Invalid used address.
     UsedAddress,
     /// Invalid features provided from vhost-user backend